Unmanned Aerial Vehicles (UAVs) are unpiloted aircraft that are either controlled remotely or are flown autonomously. UAVs are commonly categorized based on their design and performance spanning the range from small low altitude to large high altitude long endurance vehicles. The UAV technology is taking an increasingly important place in our society for commercial, civilian and military applications. UAVs could provide improved service over existing systems in a large number of applications, ranging from border patrol and coastal surveillance, monitoring of natural disasters, meteorology and cartography to highly flexible telecommunication relay stations. The required endurance may be in the range of a few hours, for example in the case of law enforcement, border surveillance, forest fire fighting or power line inspection. Other applications at high altitudes, such as, for example, communication platform for mobile devices, weather research and forecast, environmental monitoring, may require remaining airborne for days, weeks, months or even years. It is possible to reach these goals using renewable energy sources.
One of the readily available renewable energy sources is sunlight. The use of sunlight as a source of energy for aircraft has many compelling advantages. Sunlight provides about 1000 W/m2 at sea level, but reaches more abundant 1400 W/m2 at high altitudes also unobstructed by cloud cover. Photovoltaic (PV) cells and modules may be used to collect the solar energy during the day, a part of which may be used directly for maintaining flight and onboard operations with the remainder being stored for the night time.
So far the solar energy has been the only renewable energy source seriously considered for use onboard UAVs. However, the atmospheric environment provides other potentially plentiful and useful sources of energy, for example wind. Unlike solar power, wind power may be available 24 hours a day. Hybrid power systems based on solar and wind power could provide UAVs with more reliable and effective renewable power sources. With advances in efficient and smart power systems, aircraft powered by renewable energy sources may achieve sustained flight at high altitudes for days, weeks and even years.